There are known microscopes based on the light-sheet illumination technique that allows excitation light to be incident on a specimen along a plane orthogonal to the optical axis of the detection optical system for detecting fluorescence from the specimen (e.g., refer to Patent Literature 1 below). According to the epi-illumination technique or the transillumination technique, two-dimensional images suffering less blurring in the optical-axis direction are acquired by two-dimensionally scanning excitation light focused on a single point or a plurality of points using a confocal optical system. On the other hand, according to the light-sheet illumination technique, the time required to acquire an image can be reduced because just the plane corresponding to the focal plane being observed needs to be illuminated in the optical-axis direction of the detection optical system whereas a wide area can be illuminated all at once in a direction orthogonal to the optical axis of the detection optical system.
Furthermore, there are also known microscopes based on the light-sheet illumination technique that allows planar excitation light beams with different wavelengths to be simultaneously incident on a specimen from two opposite directions of the specimen along a plane orthogonal to the optical axis of the detection optical system for detecting fluorescence from the specimen (e.g., refer to Patent Literature 2 below). The microscope described in Patent Literature 2 allows excitation light beams with different wavelengths to be simultaneously incident on the same region of the same specimen from opposite directions. Because fluorescences generated in response to the excitation light beams have different wavelengths from one another, two or more fluorescence images can be acquired at a time using the same detection optical system and can be separated, that is, classified by wavelength.